US President addresses UN summit in New York, vowing to "do what we must" to stop Tehran from acquiring nuclear arms.

Barack Obama, the US president, has addressed the UN general assembly in New York, where he said that he wants to resolve the dispute over Iran's nuclear programme through diplomacy but that the time to do so is "not unlimited".

Obama said in his speech to world leaders on Tuesday that Iran has repeatedly failed to demonstrate that its nuclear programme is peaceful and has failed to meet its obligations to the United Nations.

He also said the Iranian government has been propping up the dictatorship in Syria and supporting terrorist groups abroad.

"The United States will do what we must to prevent Iran from obtaining a nuclear weapon,'' the president said.

This is Obama's final international address before the November elections.

He also insisted there has been "progress" since the Arab Spring but said the recent turmoil in the Muslim world showed the hard task of achieving true democracy.

"The events of the last two weeks speak to the need for all of us to address honestly the tensions between the West and an Arab World moving to democracy," Obama said.

Syria 'calamity'

Opening the meeting on Tuesday, UN Secretary-General Ban Ki-moon described the fighting in Syria as "a regional calamity with global ramifications".

"It is the duty of our generation to put an end to impunity for international crimes in Syria and elsewhere."

- Ban Ki-moon, UN secretary-general

In his annual state of the world speech at UN general assembly's ministerial meeting, Ban said he was sounding the alarm about widespread insecurity and injustice, inequality and intolerance in many countries.

Putting the spotlight on Syria, Ban said "the international community should not look the other way as violence spirals out of control''.

Ban said the 18-month conflict was a growing threat to international peace that required attention from the deeply divided UN Security Council.

He also expressed "profound concern'' at continuing violence in Afghanistan and Congo, unrest in Africa's Sahel region, the dangerous Israeli-Palestinian impasse and "shrill war talk'' from Israel.

'Brutal' rights abuses

"The international community should not look the other way as violence spirals out of control," Ban told world leaders, adding that "brutal" rights abuses were being committed by President Bashar al-Assad's government.

"I call on the international community - especially the members of the Security Council and countries in the region - to solidly and concretely support the efforts" of UN-Arab League envoy Lakhdar Brahimi.

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"We must stop the violence and flow of arms to both sides and set in motion a Syrian-led transition as soon as possible," Ban added.

Russia, Assad's main ally, and China have vetoed three Security Council resolutions which could have led to sanctions against the Syrian government.

Ban said the crimes being committed in Syria must not go unpunished. "There is no statute of limitations for such extreme violence," he insisted.

"It is the duty of our generation to put an end to impunity for international crimes in Syria and elsewhere."

Ban said he wanted his address to the 193-member UN to "sound the alarm about our direction as a human family".

The UN secretary-general condemned governments which spend "vast and precious funds on deadly weapons" at a time of growing climate change, economic crisis and growing poverty.

'Dangerous impasse'

He said that "the door may be closing for good" on chances of creating separate Palestinian and Israeli states because of the growth of Israeli settlements in the occupied territories.

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"We must break this dangerous impasse," he said.

Ban called the anti-Islam internet video which has inflamed protests across the world "a disgraceful act of great insensitivity" which he said had caused "justifiable offence and unjustifiable offence".

"I am profoundly concerned about continued violence in Afghanistan and in the Democratic Republic of Congo," he said."The crisis in the Sahel is not getting sufficient attention and support."

"Poverty, fragility, drought and sectarian tensions are threats to stability across the region," Ban said. "Extremism is on the rise," he warned, referring to the Islamist groups who have seized northern Mali.

"The international community needs a major concerted effort to address this alarming situation," he added.

The UN leader called on Sudan and South Sudan to resolve all the differences that brought them to the verge of war this year.